The present invention relates to an electron microscope having an X-ray detector for detecting the X-rays produced from a specimen and, more particularly, to an electron microscope having the above-described X-ray detector inserted between the upper and lower magnetic pole pieces of the objective lens.
In transmission electron microscope, a specimen is placed between the upper and lower magnetic pole pieces in the electron beam path. Where an elemental analysis of the specimen is made, an X-ray spectrometer is added to detect and analyze the X-rays emitted by the specimen in response to the incident electron beam.
X-ray spectrometers are classified into two major categories: (1) wavelength-dispersive spectrometer which uses a diffracting crystal to disperse and analyze the X-rays coming from a specimen and (2) energy-dispersive spectrometer consisting of a semiconductor detector detecting the X-rays emanating from a specimen and a pulse height analyzer that analyzes the amplitudes of the output pulses from the semiconductor detector.
An electron microscope equipped with such an energy-dispersive spectrometer is disclosed in, for example, U.S. Pat. No. 3,924,126. FIG. 1 is a cross-sectional view of an electron microscope having an energy dispersive spectrometer. FIG. 2 is a cross-sectional view taken on line A-A of FIG. In both Figs. 1 and 2, a specimen 4 is disposed on the axis Z of an electron beam between the upper magnetic pole piece 2 and the lower magnetic pole piece 3 of an objective lens. The electron beam falls on the specimen 4 at a point 0. Characteristic X-rays X emerging from the point 0 enter a circular semiconductor detector 5 and are detected. The detector 5 is mounted in the space between the pole pieces 2 and 3 in an opposite relation to the point 0. The detector 5 is mounted on the front end surface of a heat transfer rod 6 which extends through a known airlock mechanism 12 into the microscope, the mechanism 12 being mounted on the outer wall 1. The other end of the rod 6 is located inside a cooling vessel 8 filled with liquid nitrogen, for example. Thus, the detector 5 is kept at a low temperature. The detector 5 and the heat transfer rod 6 are surrounded by a cylinder 9 whose on end is situated inside a vacuum chamber 10 in which the cooling vessel 8 is housed. The front end of the cylinder 9 is sealed off by a material transmitting X-rays, such as a beryllium thin film 11, to maintain the inside of both vacuum chamber 10 and cylinder 9 as a vacuum.
In order to detect the characteristic X-rays emitted from the specimen 4 by the detector 5 with improved sensitivity, it is necessary to (a) increase the area of the incident surface of the detector 5 and to (b) place the detector 5 as close to the specimen 4 as possible.
In the known instrument as shown in FIG. 1 in order to bring the detector 5 close to the specimen for enhancing the sensitivity, it is required to decrease the diameters of the detector 5, the heat transfer rod 6, and the cylinder 9. Unfortunately, this leads to a reduction in the sensitivity.
If the diameter of the detector 5 is increase to increase the area, then it is necessary to place the cylinder 9 remotely from the specimen 4 to prevent the magnetic pole pieces 2 and 3 from coming into contact with the cylinder 9. However, this results in a decrease in the sensitivity.
FIG. 3 shows a conventional system in which the x-rays emanating from a specimen held horizontally are detected. It is important for this system that (c) the take-off angle .alpha.of X-rays be large, as well as the fulfillment of the aforementioned conditions (a) and (b), to enhance the sensitivity.
To satisfy the condition (c), the detector 5 and the cylinder 9 are placed close to the inclined surfaces of the detector 5. Placing the detector 5 closer to the specimen requires that the detector 5, the heat transfer rod 6, and the cylinder 9 have less diameters as indicated by the broken line a, which causes a reduction in the sensitivity.
If the diameter of the detector 5 is increased, the cylinder 9 must be positioned remotely from the specimen 4 to prevent the upper magnetic pole piece 2 from contacting the cylinder 9 as indicated by the broken line b. This leads to a decrease in the sensitivity. In this way, the conventional devices where a circular detector is mounted at the front end of a heat transfer rod are unable to offer sufficiently high sensitivity.